Histologic and ultrastructural studies were made of the cardiac morphologic alterations induced cutely in miniature swine by the administration of minoxidil, a vasodilating antihypertensive agent. Two types of lesions were observed: l) hemorrhagic lesions, related to drug-induced vascular injury and localized in epicardial and subepicardial arterioles, and 2) papillary muscle necroses, related to ischemic injury from hypoperfusion during minoxidil-induced tachycardia and hypotension.